The present invention relates to a light-amount adjusting apparatus used for optical apparatuses such as an interchangeable lens, a video camera, a digital still camera and a projector.
Cameras and interchangeable lenses are provided with an aperture stop apparatus (light-amount adjusting apparatus) whose aperture diameter can be changed to adjust a depth of field and a light amount reaching on an image-pickup plane of the image-pickup element, a film or the like. Many projection optical systems of projectors that project light from an image modulation element such as a liquid crystal panel are also provided with a similar type of light-amount adjusting apparatus.
Such a light-amount adjusting apparatus includes a so-called iris diaphragm whose aperture diameter can be changed by rotation of a plurality of (equal to or more than three) stop blades, like an iris of human eye. A boss portion and a hole portion for making a rotation center portion and a driven portion receiving a driving force are formed at a base portion of each stop blade used for the iris diaphragm.
Moreover, in each stop blades, the blade portion located further away than the base portion from the boss portion overlaps another stop blade in an optical axis direction to form the aperture that allows light to pass through.
In the conventional stop blades, the boss portion is formed of a metal boss member mechanically caulked on a metal sheet or a plastic sheet forming the blade portion and the base portion. Alternately, resin is outsert-molded to form the boss portion on the sheet. Thus, the stop blade needs many manufacturing steps, and the manufactured stop blade includes a problem in reliability.
To address these problems, Japanese Patent Laid-Open No. 6-317826 discloses a method for manufacturing a stop blade including the boss portion, in which the manufacturing steps thereof is significantly reduced by integrally forming the base portion and the boss portion with injection molding.
However, in the method for manufacturing the stop blade disclosed in Japanese Patent Laid-Open No. 6-317826, flow of resin for injection molding worsens as the thickness of the stop blade is reduced, thereby causing defects that dimension accuracy is reduced or a burr is easily generated on a peripheral portion of the stop blade. In contrast, a burden for driving the stop blade is increased as the thickness of the stop blade is increased, which increases the size of the aperture stop apparatus in its thickness direction. Moreover, a step formed between the stop blade and the aperture becomes larger when the thickness of the stop blade is increased, which causes a so-called small-aperture diffraction that deteriorates optical performance.
Further, as disclosed in Japanese Patent Laid-Open No. 2006-84658, the stop blade used for the light-amount adjusting apparatus described above is molded with resin using an injection mold constituted by a fixed mold and a movable mold.
Forming surfaces of the fixed mold and the movable mold have a fine rough shape. With these forming surfaces, a non-smooth shape (matt surface shape) for diffusely reflecting the light is transferred on both of the front and back surfaces and the peripheral end surface of the stop blade. The matt surface prevents the light coming from an object and reflected at the stop blade from becoming unnecessary light such as ghost.
However, in the stop blade disclosed in Japanese Patent Laid-Open No. 2006-84658, the fixed mold and the movable mold have a rough surface for transferring the matt surface shape to the stop blade also at their boundary (part forming the peripheral portion of the stop blade), so that the rough surfaces of these molds which are put together to form the peripheral portion of the stop blade generate the burr at the peripheral portion.
FIG. 20 shows a manufacturing state in which the stop blade disclosed in Japanese Patent Laid-Open No. 2006-84658 is formed. Reference symbol P represents a boundary line (parting line) between the movable mold and the fixed mold, and a hatched portion represents a stop blade 53 to be molded. Rough surfaces 51 and 52 respectively formed in the fixed mold and the movable mold are put together at their boundary T. FIG. 21 is an enlarged view of the boundary T shown in FIG. 20.
As shown in FIG. 21, a burr 53a is generated at the peripheral portion of the stop blade. The burr 53a is generated by resin flowing into a narrow space formed by the rough surfaces of the movable mold and the fixed mold that are put together at the boundary, the burr 53a extending in the same direction as that of the parting line P.
The burr thus generated at the peripheral portion of the stop blade that determines the aperture diameter makes an area of a region through which the light actually passes smaller than a designed value of the aperture diameter. In addition, light entering a thin translucent burr is refracted at an unexpected angle, thereby resulting in unnecessary light.
Moreover, when the burrs on the stop blades make contact with each other in the assembling or operation of the aperture stop apparatus, the burrs may be removed from the stop blade to generate dust.